$(2)$ A Temporary Bench Mark $($TBM$)$ B is needed to be set up on a tower. There is a Bench Mark A nearby with a known elevation of $19\mathrm{.}475$mAOD, as shown in Figure Q$1$; therefore, a level is set up in between to set the TBM$.$ If the elevation of TBM needs to be set as $20\mathrm{.}500$ mAOD, and the reading from the levelling staff held at $A$ is $1\mathrm{.}753m,$ what will be the reading expected from the levelling staff held on B$?$

$[2/5$ marks$]$

Figure Q$1$